Objective : We have previously shown in vitro differentiation of monocytes into osteoclast-like multinucleated cells by granulocyte-macrophage colony-stimulating factor (GM-CSF), well-known as the potent inhibitor of osteoclastogenesis. We compared GM-CSF-induced multicleated cells with those induced by receptor activator of nuclear factor kappa B ligand (RANKL) to elucidate the attributes of osteoclast-like cells generated under the pathgenic condition of rheumatoid arthritis (RA).Methods : The precursors were produced by the co-culture of monocytes with synovial nurse cells derived fro RA joints (RA-SNCs) and then subjected to the formation of multinucleated cells. Reverse transcription-polymerase chain reaction (RT-PCR) was performed to measure the expression of matrix metalloproteinase (MMP)-12 and six osteoclast markers ; tartrate-resistant acid phosphatase (TRAP), crbonic anhydrase, cathepsin K, calcitonin receptor, integirin αV and MMP-9. TRAP and MMP-12 in RA joints specimen were evaluated histochemically.Results : The mononuclear cells co-cultivated with RA-SNCs expressed ostoclast markers except for MMP-9, and maintained the ability of osteoclast-like cell formation induced by RANKL, RANKL-induced cells expressed MMP-9 as well as other ostoclast markers, but not MMP-12. On the other hand, by thestimulation of GM-CSF, the mononuclear cells also differentiated into osteoclast-like multinuleated cells expressing MMP-9 and MMP-12. Farthermore, the multinucleated cells producing MMP-12 were found in the erosive bone area of rheumatoid joints.Conclusion : These findings suggest the presence of two distinct types of TRAP-positive multinucleated cells in RA joints. GM-CSF-induced cells, distinguishable from RANKL-induced cells by the expression of MMP-12, should be involved in the pathology of RA. The co-culture with RA-SNCs is very useful system to investigate mechanism of the differentiation of synovial macrophages and their roles in the pathogenesis of RA.